1. Field of the Invention
This invention relates to modified ethylene/n-butyl acrylate (EnBA) copolymers having improved adhesion to non-polar substrates. Significantly improved metal adhesion is obtained by grafting a half-ester of maleic acid, e.g., monopropyl maleate, to EnBA copolymers.
2. Description of the Prior Art
The ability of ethylene to be polymerized with a variety of monomers, such as vinyl esters, is well known. For example, vinyl acetate (VA) is polymerized with ethylene to produce ethylene/vinyl acetate (EVA) resins which are widely used in hot-melt and other adhesive systems. It is also known that ethylene can be copolymerized with n-butyl acrylate (nBA) to produce EnBA copolymers. While EnBA and EVA resins containing comparable molar amounts of the comonomer exhibit minor differences in polarity, density, tensile, elongation, softening point, heat stability, low temperature flexibility, etc., the resins are sufficiently similar so that they can be used in many of the same Comparisons of EVA and EnBA resins of the type used in hot melt adhesive systems are discussed in an article by D. C. Wielinski, Adhesives Age, November 1989, pp. 30-33.
Even though EVA and EnBA resins are useful for the formulation of hot melt adhesive and sealant systems, adhesion to metals, such as aluminum, is generally considered to be quite low. We have now found that significant improvement of metal adhesion can be achieved by crafting a half-ester of maleic acid, such as monopropyl maleate, to EnBA copolymers. While it is generally known that adhesion to metals and other non-polar substrates can be enhanced by incorporating adhesion promoting carboxylic monomers, the improvement obtained with the present invention is unexpected compared to the results obtained with comparable grafted EVA copolymers.
Numerous adhesion promoting monomers, primarily acrylic acid and maleic acid (or derivatives thereof), have been incorporated into ethylene copolymers either by direct polymerization or grafting. Ethylene/vinyl acetate/methacrylic acid terpolymers, for example, are known and commercially available from E. I. duPont le Nemours and Company under the trademark Elvax. Ethylene/monomethyl maleate/alkyl acrylate polymers are disclosed in U.S. Pat. Nos. 2,599,119 and 2,599,123. Ethylene and monomethyl maleate are copolymerized in British Pat. 963,380 and copolymers of ethylene and monobutyl maleate are disclosed in Canadian Pat. 1,118,141. Polymers of monoalkyl esters of maleic acid and vinyl acetate, with and without other comonomers, are disclosed in U.S. Pat. Nos. 4,013,805, 4,116,913, 4,347,341, and 4,599,378.
Numerous references are also known which disclose grafting of unsaturated polycarboxylic compounds and their partial or complete esters to a variety of polymer substrates. Generally, grafting is accomplished using peroxide but some processes use other radical-generating means. For example, polypropylene and propylene/.alpha.-olefin copolymers are thermally degraded and grafted in U.S. Pat. Nos. 3,480,580 and 3,481,910. Mono- and diesters of maleic acid are disclosed as suitable grafting monomers in both references. Maleic semiesters and semiamides are disclosed for grafting a variety of polymer substrates, including polyethylene and EVA, in U.S. Pat. No. 4,347,341.